1. Operating torque: The operating torque is the most important parameter for selecting the valve electric device. The output torque of the electric actuator should be 1.2 to 1.5 times the maximum operating torque of the valve.
2. There are two main structures for operating the thrust valve electric device: one is not equipped with a thrust plate, and the torque is directly output; the other is with a thrust plate, and the output torque is converted into output thrust through the stem nut in the thrust plate.
3. Number of rotations of the output shaft: The number of rotations of the output shaft of the valve electric device is related to the nominal diameter of the valve, the pitch of the valve stem, and the number of thread heads. It should be calculated according to M=H/ZS (M is the electric device should meet The total number of turns, H is the valve opening height, S is the thread pitch of the valve stem, and Z is the number of stem threads).
4. Valve stem diameter For multi-turn rising stem valves, if the maximum stem diameter allowed by the electric device cannot pass through the stem of the equipped valve, it cannot be assembled into an electric valve. Therefore, the inner diameter of the hollow output shaft of the electric device must be larger than the outer diameter of the stem of the rising stem valve. For part-turn valves and dark-stem valves in multi-turn valves, although the passage of the stem diameter does not need to be considered, the stem diameter and the size of the keyway should be fully considered when selecting and matching, so that it can work normally after assembly.
5. Output speed: If the opening and closing speed of the valve is too fast, water hammer is easy to occur. Therefore, the appropriate opening and closing speed should be selected according to different conditions of use.
6. The valve electric actuator has its special requirements, that is, it must be able to limit the torque or axial force. Usually the valve electric device adopts a torque-limiting coupling. When the specifications of the electric device are determined, its control torque is also determined. Generally, it runs within a predetermined time and the motor will not be overloaded.
However, overload may result in the following situations:
One is that the power supply voltage is low and the required torque cannot be obtained, causing the motor to stop rotating;
The second is to set the torque limit mechanism incorrectly to make it larger than the stopping torque, resulting in continuous generation of excessive torque and stopping the motor;
The third is intermittent use, and the heat accumulation generated exceeds the allowable temperature rise of the motor;
Fourth, the circuit of the torque limiting mechanism fails due to some reason, causing the torque to be too large;
Fifth, the use environment temperature is too high, which relatively reduces the thermal capacity of the motor.
In the past, the way to protect the motor was to use fuses, overcurrent relays, thermal relays, thermostats, etc., but these methods have their own advantages and disadvantages. There is no absolutely reliable protection method for variable load equipment such as electric devices. Therefore, various combinations must be adopted, which can be summarized in two ways: one is to judge the increase or decrease of the input current of the motor; the other is to judge the heating of the motor itself. Regardless of these two methods, the time margin given by the motor’s thermal capacity must be considered.
Generally, the basic protection method for overload is:
1. Use a thermostat for overload protection of the motor for continuous operation or jog operation;
2. The thermal relay is used to protect the motor from stalling;
3. For short-circuit accidents, use fuses or overcurrent relays.
The valve electric actuator is an indispensable device to realize valve program control, automatic control and remote control. Its movement process can be controlled by the stroke, torque or axial thrust.